Add RpcAgentTestFixture to extract duplicate code (#29747)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29747
There are duplicate code for component that rely on RpcAgent. Extract them into a re-usable test fixture class.
Test Plan:
### RPC + RRef
```
buck test mode/dev-nosan //caffe2/test:rpc_fork
buck test mode/dev-nosan //caffe2/test:rpc_spawn
```
```
buck test mode/dev-nosan //caffe2/test:rpc_fork_thrift
buck test mode/dev-nosan //caffe2/test:rpc_spawn_thrift
```
### Dist Autograd
```
buck test mode/dev-nosan //caffe2/test:dist_autograd_fork
buck test mode/dev-nosan //caffe2/test:dist_autograd_spawn
```
```
buck test mode/dev-nosan //caffe2/test:dist_autograd_fork_thrift
buck test mode/dev-nosan //caffe2/test:dist_autograd_spawn_thrift
```
### Dist Optimizer
```
buck test mode/dev-nosan //caffe2/test:dist_optimizer_fork
buck test mode/dev-nosan //caffe2/test:dist_optimizer_spawn
```
```
buck test mode/dev-nosan //caffe2/test:dist_optimizer_fork_thrift
buck test mode/dev-nosan //caffe2/test:dist_optimizer_spawn_thrift
```
Differential Revision: D5689636
fbshipit-source-id: f35eea1359addaaac9bd8d00d0a5df228a236511
